Fire-retardant polymer foam materials are often used in packaging applications. They are commonly used in the form of sheets, for example, to wrap objects and, thus, to protect the wrapped objects from damage caused by impact or abrasion during shipping.
There are a variety of approaches to making fire-retardant polymeric materials, many of which can be used to make polymer foam materials. One approach is to compound fire-retardant additives with the polymer during foam fabrication. In another approach, fire-retardant additives are applied as a coating to the pre-formed polymer foam material. These technologies are reviewed in Kirk-Othmer Encyclopedia of Chemical Technology, John-Wiley (New York, 3d ed. 1978) and in The Encyclopedia of Polymer Science and Engineering, John-Wiley (New York, 2d ed. 1985).
A disadvantage of the first of the aforementioned approaches is the perturbation of the manufacturing process in making the polymer foam material. As discussed below, the use of additives with a polymer makes the properties of the material less suitable in the preferable foam manufacturing process. The alteration of the material properties may also render the polymer foam less desirable as a final product. Those disadvantages are avoided by the present invention.
In the commercial manufacture of, for example, polypropylene foam packaging materials, the foam is extruded and formed as the polymer solution exits the extrusion device. The use of fire-retardant additives in the polymer solution can deleteriously affect the foam extrusion manufacturing process or the properties of the final foam product. For example, if the additives do not totally dissolve in the polymer solution before extrusion, the undissolved additives can act as nuclei which alter the cell structure of and weaken the foam.
On the other hand, to the extent that fire-retardant additives do dissolve in the polymer solution before extrusion, the additives alter the vapor pressures of the blowing agents and inflatants used in the foam extrusion process. This can adversely affect the manufacturing process and the final product, requiring major processing changes to produce an acceptable fire-retardant foam product. In addition, many fire-retardant additives, including the most common and least expensive halogenated organic additives, are unstable under the processing conditions used to make certain foams. Thus, they could not be used; rather, their less common and more expensive counterparts would be required.
The second alternative approach to constructing fire-retardant polymeric materials, using fire-retardant additives and a binder to coat the polymer foam, presents its own problems. These coatings can adversely affect the "hand" of the polymer foam composite material, for example, because the additives appear on the external surface of the material. Similarly, the fire-retardant coating can flake or rub off onto the packaged objects, or possibly onto the hands of those handling the product--creating health and safety concerns.
In the manufacture of thick foam sheets, regardless of their fire-retardant properties, a separate bonding step is often necessary. The bonding step builds a thick foam sheet composite from the separate, thinner sheets after the thinner sheets have been extruded. The extruded sheets are laminated, or layered, and bonding can be achieved using a variety of techniques--including hot pin perforation or the application of conventional adhesives. As a method of forming a fire-retardant polymer foam composite, a one-step extrusion lamination technique is preferable: the composite layers bond together upon extrusion, as part of the process for incorporating the fire-retardant additives, and a separate bonding step to increase thickness is therefore unnecessary.
To overcome the shortcomings of existing fire-retardant polymeric materials, a new polymer foam composite is provided. An object of the present invention is to provide an improved polymer foam composite without perturbing the manufacturing steps used to make the foam itself. A related object is to avoid the need for the separate bonding steps often used to build thicker, multiply foam products from thinner piles. Another object is to incorporate the fire-retardant additives without interfering with the appearance, feel, frictional properties, and safety in handling of the foam.
It is still another object of the present invention to incorporate relatively inexpensive additives, in lesser amounts, than used by conventional coated products--while improving (by reducing) flame spread and smoke density test results. An additional object is to improve the physical properties of the composite, such as puncture resistance and tear strength. Yet another object of this invention is to allow manufacturing flexibility by using existing extrusion-lamination equipment, currently used to make similar products, for making the polymer foam composite of the present invention. The need for special coating and drying equipment is thereby avoided.